Fabrics made of polyester multifilaments are in wide use as various cloth materials for sportswear, casual wear, coats and the like or as clothes such as downproof fabrics, and umbrella cloths. So far, polyester multifilaments having a thickness of about 56 dtex/48 f have been used as such polyester multifilaments. In recent years, however, lightweight polyester fabrics lighter in weight, higher in density and flexible have been demanded in various fields of application.
In the case of downproof fabrics to be used in down jackets or as bedclothes, for instance, polyester fabrics light in weight, high in density and flexible are desired so that high levels of warmth retention may be obtained. The use of such polyester fabrics makes it possible to obtain warmth retaining materials low in air permeability and excellent in warmth retention. Furthermore, since the resulting fabrics are flexible, cotton, down and the like, which are used in down jackets, bedclothes and so forth as warmth retaining mass, will not be excessively compressed by the downproof fabrics; hence, the air retention rate can be increased. Owing to such effects, still better warmth retention can be attained.
Further, in the field of umbrella cloths, fabrics high in density, excellent in waterproofness and, at the same time, excellent in portability are desired. When polyester fabrics light in weight, high in density and flexible are used, umbrellas excellent in portability can be obtained because the cloths are high in density and thin and therefore are reduced in volume and the umbrellas can be reduced in volume upon folding. The conventional folding umbrellas have a problem, namely the umbrella cloth is poor in flexibility, the folding parts tend to hold air excessively and, unless folded along the folding lines provided in advance, the umbrellas cannot be sufficiently reduced in volume and, as a result, time and effort are required for folding them.
In addition, in the fields of tent cloths, casual wear, coats and the like, the use of polyester fabrics light in weight, high in density and excellent in flexibility is preferred since such favorable characteristics as portability, warmth retention, lightness in weight, and feel and touch can also be then obtained.
A known method of rendering cloths lightweight comprises using hollow fibers, for instance (Japanese Kokai Publication 2002-309463). However, although they are lightweight, such hollow fibers have problems, namely they are unsatisfactory in thinness and lacking in portability and compactness. In addition, hollow fibers are weaker in tear strength as compared with ordinary fibers on the same fineness level.
As another method of rendering cloths lightweight, there may be mentioned the method which uses extra-fine polyester multifilaments which are finer than the conventional polyester multifilaments. However, fabrics made of extra-fine polyester multifilaments are insufficient in tear strength and therefore cannot have those performance characteristics which make fabrics suited for use in the above-mentioned various fields of application. Further, the difficulty in obtaining flexibility is another problem.